The invention relates to a bone screw for connection at different angles to a longitudinal bar for osteosynthesis, in particular to a pedicle screw for implants for correcting and stabilizing a spinal column, with a threaded shaft for screwing into the bone, in particular into a vertebra, and with a screw head made for securing to a support bar.
Such bone screws are known in the prior art, having in order to set one of a plurality of possible angles a set of teeth formed symmetrical to the pivot axis so that they are very expensive and difficult to manufacture and only allow settings to be made at predetermined angular offsets.
Pedicle screws are known for example from German 4,107,480 that have a threaded shaft and a two-part head that fit together via a dovetail-shaped guide. The screw head is formed by a yoke in which the support bar is fixed after it is slipped into the dove tail and secured by a set screw. Such pedicle screws have shown themselves to be effective in practice; however it would be preferable after screwing the pedicle screw into the body of the vertebra if the position on the support bar and the fitting could be done with fewer parts and operational steps.
It is therefore an object of the invention to provide a bone screw of the above-described type that can be freely adjusted after being fitted to the support bar along the axis of same.
This object is achieved by a bone screw of the above-described type wherein the screw head is formed as a partial sphere with a frustoconically tapered peg having a threaded bore, the tapered peg with its threaded bore extends at an acute angle to a longitudinal axis of the threaded shaft, and the tapered peg is securable by means of a screw engaging in the threaded bore to a clamping sleeve forming a seat for the support bar.
The invention has the advantage that when the screw is loosened the clamping sleeve can be rotated on the peg and the support bar can be slid longitudinally through the seat of the sleeve so that the operator can simply adjust the orientation as well as the relative positions of the bar on the bone. As soon as the desired position with any angular setting is reached, tightening of the screw ends both the angular and longitudinal freedoms of movement. No teeth are needed because the peg has a surface inclined relative to the rotation axis of the screw engaging in the threaded bore so that the load moment of the connection between the clamping sleeve and the bone screw does not have to be borne only by friction or engagement with teeth, but instead is distributed over the entire surface of the peg and is borne by bending stress in the clamping sleeve and in the support bar.
Preferably the tapered peg is surrounded on the partial sphere by a support edge for the clamping sleeve so that there is more than engagement only with the flanks of the tapered peg.
A preferred embodiment of the invention is characterized in that the clamping sleeve is formed by a bendable clip having parallel legs connected together by a spring ring forming the seat and having holes through which the screw engages. This embodiment has the advantage that at different stages of tightening of the screw the pivotability of the clamping sleeve or the axial slidability of the support bar can be limited or blocked.
It has shown itself advantageous when the walls of the holes have the same angle as that of the tapered peg so that there is not only line contact with high pressure, but a larger surface area is available.
When the outer wall of the tapered peg has a circumferential outwardly open recess it is possible that the angle of the clamping sleeve can be changed at least limitedly before tightening the screw.
In order to maximize the surface of the abutment edge, the partial sphere is formed by a half sphere merging with the threaded shaft.